1. Field of the Invention
The present invention generally relates to inspection systems for inspecting leads on circuit packages and, more particularly, to systems that inspect the leads on circuit packages of the surface-mounted type.
2. State of the Art
In the assembly of printed circuit boards, the fraction of assembled boards which meet performance specifications is known as the "yield percentage". The goal in most assembly operations is to maximize yield percentage subject to constraints on assembly time. To meet that goal, assembly conditions must be such as to avoid circuit malfunctions on assembled boards.
For assemblies employing integrated circuit packages of the surface-mounted type, circuit malfunctions often can be traced to faulty solder connections between a board and the leads of a circuit package mounted on the board. Typically, leads on circuit packages of the surface-mounted type are finer and more closely spaced than leads on circuit packages of the dual in-line pin (DIP) type. Circuit packages of the DIP type have leads that are for insertion into holes in a circuit board for purposes of making electrical connections, whereas circuit packages of the surface-mounted type are designed, as the name implies, for electrical connection to the surface of a circuit board. Circuit packages of the surface-mounted type are normally rectangular and have leads on two or all four sides.
For mounting packages of the surface-mounted type to a circuit board, solder paste is silk-screened onto the minute solder pads that are precisely located on the board to match the pattern or "footprint" of properly aligned leads on the circuit package. After the desired number of circuit packages are placed on the board with the package leads embedded in the solder paste on the pads, the packages are permanently soldered into place. To achieve satisfactory electrical connections on circuit boards using surface-mounted packages, not only must the lead patterns of the packages match the pattern of pads on a circuit board but, also, the contact ends of the leads of a circuit package must all lie substantially in a common plane. This latter condition is often expressed by saying the contact ends of leads of surface-mounted packages must be substantially coplanar. Typically, specifications for surface-mounted packages have required that the contact ends of leads are coplanar within less than about two to about four thousandths of an inch (mils).
As used above in connection with circuit packages of the surface-mounted type, the term "contact end" refers to the portion of a package lead which does, or is intended to, contact a solder pad on a circuit board when the leads are properly aligned (e.g., not bent) and when the package is placed in its normal position. Equivalently, the term "contact end" can be understood to refer to the portion of a circuit package lead which is closest to a flat reference surface when the circuit package rests in its normal position on the reference surface. It should be noted that the actual end of a lead can be significantly different from the contact end, since leads of surface-mounted packages can have various shapes including "J" and gull-wing like shapes. It should also be noted that, when a circuit package is viewed from the side, the contact end of a lead appears to have an edge; such an edge will be referred to herein as an "apparent edge".
Currently, several methods are used for detecting whether the contact ends of leads on surface-mounted circuit packages are coplanar. The conventional detection methods include reflected image comparisons, image analysis employing video cameras, transmitted light measurements, laser-based dimensional measurements, and cast-shadow analysis. As will be discussed below, each of these inspection methods has substantial drawbacks.
The first inspection method, reflected image comparison, is performed with the aid of optical microscopes. Typically in this method, a circuit package is rested on a reflecting surface with the contact ends of the leads supporting the package. Using a microscope at a viewing angle close to parallel to the reflecting surface, each contact end and its reflected image are inspected. When so viewed, the apparent gap between the contact end of a lead and its reflected image is twice the actual gap. To detect whether leads of a surface-mounted circuit package are coplanar by the method of reflected image comparisons, all of the gaps at the contact ends of the leads are measured and then compared. Such manual measurements may require up to several minutes per package, depending upon the number of leads on a package.
It is also known to use video cameras in systems that inspect the leads on circuit packages. In this context, the term video camera refers to cameras that depict a scene in two dimensions. The use of a video camera in an automatic inspection system for inspecting rows of pins on integrated circuit packages of the DIP type is described in U.S. Pat. No. 4,696,047. In the patented inspection system, a conveyor moves the DIP packages past a video camera which provides two-dimensional images of the pins. The images are analyzed by a signal processor which compares the images with a reference stored in the memory of a digital computer. Based upon the image comparisons, the inspection system rejects circuit packages whose pins do not meet specified tolerances.
Also, U.S. Pat. No. 4,668,939 discloses an automated system using a video camera for inspecting solder bumps on chip carriers. According to the patent, the images provided by the video camera are processed to provide one-dimensional intensity plots. The patent states that the intensity plots can be analyzed to detect missing, bridged or excessive solder bumps.
Image analysis employing conventional video cameras has several disadvantages. One difficulty arises from the fact that conventional cameras provide two-dimensional video information which is arranged in rectangular rasters, typically having aspect ratios of about 4:3. By way of contrast, the region of interest when inspecting leads of circuit packages of the surface-mounted type can be, for example, about two-thousand mils wide by twenty mils high, which equates to an aspect ratio of about 100:1. (In this example, the two-thousand mil dimension would represent the length of a surface-mounted package, and the twenty mil dimension would represent the area of interest for encompassing the space between the contact end of a lead and the reference surface.)
To increase aspect ratios when inspecting leads on circuit packages of the surface-mounted type with a video camera, the camera can be fitted with a cylindrical lens that provides image magnification in the vertical direction greater than in the horizontal direction. The purpose of such differential magnification is to increase picture element resolution in the vertical direction while retaining sufficient horizontal field width to allow viewing of an entire side of a circuit package. (As to picture element resolution, it may be noted that video cameras that operate according to conventional television standards, for example, provide about five-hundred elements both horizontally and vertically, for a total of about 250,000 elements in a two-dimensional scene.) Even with cylindrical lenses, however, conventional video cameras may not provide adequate resolution in both the vertical and horizontal dimensions for inspection of the leads of circuit packages of the surface-mounted type.
In a somewhat different technique for inspecting leads on surface-mounted circuit packages, fiber-optic sensors have been used to make so-called "transmitted light" measurements. According to this technique, when a circuit package stands on its leads on a planar surface, a gap between the contact end of a lead and the planar surface can be considered to be analogous to a gate. By this analogy, the quantity of light transmitted through a given gate is directly proportional to the opening provided by the gate, which is to say that the transmitted light is proportional to the gap between the contact end of an inspected lead and the reference surface. By measuring the light intensity at the gap for each lead, one can determine the extend to which the contact ends of the leads of a circuit package are coplanar.
One drawback of transmitted light measurements, however, is that a complete sensor head must be custom designed and fabricated for each package configuration and the conversion of the system from one package type to another is difficult and time-consuming. Another drawback of transmitted light measurement is that such measurements of leads of circuit packages of the surface-mounted type can vary depending upon whether the apparent edges at the contact ends of the leads are horizontal or angled from horizontal. When the contact ends of leads have substantially angled edges, measurements of transmitted light intensity indicates the average distance from a contact end to the reference surface. It should be understood that such average measurements are usually of limited value in determining whether the contact end of a lead will, in fact, adequately contact a solder pad on a circuit board.
It is also known to use laser beams in systems that detect whether the contact ends of leads on surface-mounted circuit packages are coplanar. In typical laser-based systems, a laser beam is directed against a mirror which rotates so that the reflected beam scans across a plane. When a surface-mounted circuit package is stationarily positioned with its leads directed upward, a succession of such laser-based scans at different elevations can provide a profile of the contact ends of the leads. A major disadvantage of using lasers to measure the elevations of the contact ends of leads on circuit packages, however, is that leads with sloping surfaces may not predictably reflect light.
In making measurements of leads on surface-mounted packages by the shadow casting technique, a light source casts shadows of the leads onto an optically flat surface. Then, a photosensor samples the intensity of light at points adjacent to and within the penumbra of each lead shadow to measure the length of the shadow of each lead. Based upon comparisons between the actual and theoretically-expected shadow lengths, the actual location of the contact ends of leads can be inferred. Shadow casting measurements are not always acceptable, however, because they are less accurate than direct measurements of lead dimensions.